Treatment stages for selenium removal

ABSTRACT

Methods and apparatus relate to treating fluid to at least reduce selenium content within the fluid. The treating includes conditioning stages to alter a composition of the fluid prior to removal of the selenium content from the fluid. The composition of the fluid after the conditioning stages facilitates the removal of the selenium content or at least limits detrimental impact to selenium removal efficiency.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of application Ser. No.13/035,372, filed Feb. 25, 2011, which claims benefit under 35 USC§119(e) to U.S. Provisional Application Ser. No. 61/307,916 filed Feb.25, 2010, entitled “TREATMENT STAGES FOR SELENIUM REMOVAL”.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

None

FIELD OF THE INVENTION

Embodiments of the invention relate to methods and systems for stagedtreating of fluid to remove selenium from the fluid.

BACKGROUND OF THE INVENTION

Fossil fuels contain naturally occurring selenium, which exists inseveral oxidation states, including selenide (−2), elemental selenium(0), selenite (+4), and selenate (+6). Refining of oils and processingof coals containing selenium can generate process water with amounts ofselenium-containing compounds in excess of limits allowed bygovernmental standards for discharge of the water into the environment.These aqueous streams often include the selenium in soluble forms, suchas selenocyanate (SeCN) in zero oxidative states and selenite (SeO₃ ⁻²)and selenate (SeO₄ ⁻²) as oxidized species.

Various treatment techniques for the process water provide ways toremove selenium and may rely on adsorption. The techniques often removeall types of the selenium to some extent but with relative less abilityto remove the oxidized species of the selenium. Past improvements toselenium removal effectiveness and efficiency focus on materials forsorbents and sorption reaction conditions between the sorbents and theselenium. However, the treatment techniques still may not enableachieving selenium limits at all times and with a sufficient margin fromdesired or regulated levels. Cleaned water resulting from the treatmenttechniques further often fails to meet criteria for reuse of the cleanedwater in such applications as feed to boilers or desalting units.

Therefore, a need, exists for improved methods and systems for removalof selenium from a fluid.

SUMMARY OF THE INVENTION

In one embodiment, a method of treating an aqueous stream includesremoving a selenium sorption inhibitor from the aqueous stream. The stepof removing the selenium sorption inhibitor includes reducing inconcentration at least one of oils, soluble organic compounds, andthiosulfate from the aqueous stream. Next, the method includes removingselenium from the aqueous stream by passing the aqueous stream fromwhich the concentration of the sorption inhibitor has been reduced intocontact with a support impregnated with at least one of sulfur, seleniumand tellurium to absorb the selenium and provide treated water aseffluent.

According to one embodiment, a system for treating an aqueous streamincludes a conditioning unit having at least one of an oil removalcomponent, a soluble organics removal component, and a thiosulfateremoval component coupled to receive the aqueous stream for treatmentthereof. The system further includes a selenium removal unit having atreated water output and an input coupled to receive an outflow of theaqueous stream from the conditioning unit. The outflow is in fluidcommunication with a support impregnated with at least one of sulfur,selenium and tellurium to absorb selenium in the outflow within a flowpath between the input and the treated water output.

For one embodiment, a method of treating an aqueous stream includesremoving selenium from an aqueous stream by passing the aqueous streamthrough a sorbent bed and into contact with a support impregnated withat least one of sulfur, selenium and tellurium to absorb the selenium.Adding a reducing agent to the aqueous stream removed from the sorbentbed and containing some of the selenium in a positive oxidation statecauses the selenium to form a precipitate. The method further includesremoving the precipitate from the aqueous stream.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further advantages thereof, may best beunderstood by reference to the following description taken inconjunction with the accompanying drawings.

FIG. 1 is a flow chart for a selenium removal process, according to oneembodiment of the invention.

FIG. 2 is a system flow diagram of functional fluid treatmentsutilizable in various combinations to form all or part of the seleniumremoval process, according to one embodiment of the invention.

FIG. 3 is a system flow diagram of alternative functional fluidtreatments that may also form all or part of the selenium removalprocess, according to one embodiment of the invention.

FIG. 4 is a block diagram for a first exemplary implementation of theselenium removal process, according to one embodiment of the invention.

FIG. 5 is a block diagram for a second exemplary implementation of theselenium removal process, according to one embodiment of the invention.

FIG. 6 is a block diagram for a third exemplary implementation of theselenium removal process, according to one embodiment of the invention.

FIG. 7 is a block diagram for a fourth exemplary implementation of theselenium removal process, according to one embodiment of the invention.

FIG. 8 is a plot of a first order rate constant for selenium removalversus water processed showing negative impact by oils on seleniumsorption performance.

FIG. 9 is a plot of a first order rate constant for selenium removalversus water processed illustrating relative to FIG. 8 sorbent lifeextended by removal of soluble organics before contact of the water withsorbent for absorption of selenium even though the oils still influencethe selenium sorption performance.

FIG. 10 is a bar graph of comparative undesirable increases in sorbentbed lengths required to obtain a 95% selenium removal threshold when thewater contains various dissolved organic compounds.

FIG. 11 is a plot showing negative influence of thiosulfate on seleniumremoval due to additional mass transfer zone (MTZ) length needed asthiosulfate concentration increases.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention relate to treating fluid to at least reduceselenium content within the fluid, which may be an aqueous liquid from apetroleum refinery, a power plant, irrigation runoff, mine tailings,solid fuel gasification or industrial waste, for example. As usedherein, “selenium” refers to selenium within or from compounds, such asselenocyanate, selenite, selenate, hydrogen selenide, organo-seleniumcompounds, and combinations thereof, containing selenium and at leastone other element and/or elemental selenium. Concentration of theselenium thus provides the selenium content within the fluid. Thetreating includes conditioning stages to alter a composition of thefluid prior to removal of the selenium content from the fluid. Thecomposition of the fluid after the conditioning stages facilitates theremoval of the selenium content or at least limits detrimental impact toselenium removal efficiency.

FIG. 1 shows a flow chart for one embodiment of a selenium removalprocess. A stream of fluid produced in a supply step 100 containsselenium. Some applications such as described further herein proceedgiven a first treatment path selection 102 to an inhibitor removal step104. Desire to remove selenium in a zero oxidation state, such asselenocyanate, from the fluid provides one criteria for the firsttreatment path selection 102. The inhibitor removal step 104 includesremoving oils, dissolved organics and/or thiosulfate from the fluid. Asdemonstrated herein by FIGS. 8-11, the oils, the dissolved organics andthe thiosulfate tend to inhibit removing of the selenium from the fluidin a subsequent selenium removal step 106. The process may proceed, ifno further treatment for selenium is desired, to an output step 112providing effluent produced in the selenium removal step 106.

In some embodiments, the process proceeds given a second treatment pathselection 108 to a precipitation step 110 where a reducing agent isadded to the fluid, which may come straight from the supply step 100 ormay have some selenium already removed after the inhibitor removal step104. Since the reducing agent causes selenite to precipitate, desire toremove selenium in a positive oxidation state from the fluid providesone criteria for the second treatment path selection 108. The seleniumremoval step 106 used following the precipitation step 110 separatessolids from liquids in the fluid and thus may utilize removal techniquesthat are alike or different from those used following the inhibitorremoval step 104. If no further treatment for selenium is desired, theprocess may proceed at this point in the process to the output step 112providing the effluent.

FIG. 2 illustrates a system flow diagram of functional fluid treatmentsutilizable in various combinations and orders to form all or part of theselenium removal process to treat a fluid. The treatments utilize any ofa solids removal unit 200, an oils removal unit 202, a soluble organicsremoval unit 204 and a thiosulfate (S₂O₃ ⁻²) removal unit 206 incombination with a selenium removal unit 208. The selenium removal unit208 thus receives the fluid after having passed through at least one ofthe solids removal unit 200, the oils removal unit 202, the solubleorganics removal unit 204 and the thiosulfate removal unit 206. One ormore of the solids removal unit 200, the oils removal unit 202, thesoluble organics removal unit 204 and the thiosulfate removal unit 206may be combined in some embodiments such that one device achieves two ormore of the functional fluid treatments. For some embodiments, thetreatments may further implement a polish treatment unit 210 downstreamof the selenium removal unit 208 for removing any residual constituentsthat might otherwise prevent recycling of the fluid.

The solids removal unit 200 at least reduces concentration of solidparticulate in the fluid. Exemplary methods employed by the solidsremoval unit 200 for reducing the concentration of the solid particulateinclude size exclusion based cartridge filtration, settling,centrifugation, dissolved gas floatation, electro-coagulation, andceramic membrane filtration. The solid particulate remaining in thefluid if allowed to enter a sorbent bed forming the selenium removalunit 208 tends to plug the sorbent bed. This plugging results inelevating a differential pressure across the sorbent bed above practicaloperation levels. Further, the solids removal unit 200 may limit foulingof the selenium removal unit 208 from oils coated on the solidparticulate that is removed from the fluid prior to the fluid beingintroduced to the selenium removal unit 208.

Inflow of the fluid into the solids removal unit 200 may contain thesolid particulate in a concentration range of 1 parts per million (ppm)to 10,000 ppm, 10 ppm to 1000 ppm, or about 500 ppm. The solidparticulate may range in diameter from 1 to 10,000 micrometers or 20 to2000 micrometers. Reduction in the particulate quantity within the fluidwith the solids removal unit 200 may result in the fluid having lessthan 50 ppm or less than 10 ppm of the particulate.

The oils removal unit 202 at least reduces concentration of free andemulsified oil in the fluid. Exemplary methods employed by the oilsremoval unit 202 for reducing the concentration of the oil includeflotation, settling, electro-coagulation, ceramic membrane filtration,organo-clay bed and solvent extraction. Since many of the exemplarymethods for the oils removal unit 202 and the solids removal unit 200are alike, a single device may define the oils removal unit 202 and thesolids removal unit 200 capable of achieving desired removal of both theoils and the particulate. The oil remaining in the fluid if allowed toenter the sorbent bed forming the selenium removal unit 208 tends tocoat sorbent material forming the sorbent bed rendering the sorbentmaterial inactive.

Initial quantity of the oil in the fluid can range from 1 ppm to 10,000ppm or be greater than 100 ppm or greater than 10 ppm. Temporaryrefinery upsets may cause the quantity of the oil in the fluid to spiketo as high as 99.9% of the fluid. Reduction in the oil within the fluidwith the oils removal unit 202 may result in the fluid having less than100 ppm or less than 10 ppm of the oil.

The soluble organics removal unit 204 at least reduces concentration oforganic soluble compounds in the fluid. The compounds may includeorganic acids or phenolic compounds, such as those having a formuladefined by C_(x)H_(y)(COOH)_(n) or C_(x)H_(y)(OH)_(n), where x, y, and nare greater than 0, including naphthenic acid, formic acid, acetic acid,propionic acid, butyric acid, phenol and cresol and their derivatives.Exemplary methods employed by the soluble organics removal unit 204 forreducing the concentration of the organic soluble compounds includesolvent extraction, precipitation, nanofiltration, ion exchange,activated carbon bed, organo-clay bed and distillation. In someembodiments, spent sorbent material that no longer provides sufficientselenium sorption and that is taken from the selenium removal unit 208protected from the organic soluble compounds forms the activated carbonbed used in the soluble organics removal unit 204. Again, removal of thesoluble organics may be accomplished in tandem with removal of thesolids and/or the oils using the methods suitable for such removal.

Initial concentration of the organic acids dissolved in the fluid mayrange between 1 ppm and 10,000 ppm or between 10 ppm and 2000 ppm. Thefluid may also contain an initial concentration between 1 ppm and 10,000ppm or between 100 ppm and 1000 ppm of the phenolic compounds dissolvedin the fluid. Reduction in the soluble organics concentration within thefluid with the soluble organics removal unit 204 may result in the fluidhaving less than 10 ppm of the soluble organics.

The thiosulfate removal unit 206 at least reduces concentration ofdissolved thiosulfate in the fluid. Nanofiltration provides oneexemplary method employed by the thiosulfate removal unit 206. In someembodiments, the nanofiltration uses a membrane that has a maximum poresize below 100 nanometers. The membrane used for the nanofiltrationrejects divalent anions, such as thiosulfate, and passes monovalentanions, such as selenocyanate.

Solids, oils and/or organic compounds treatment ahead of suchnanofiltration membrane prevents the nanofiltration membrane from beingoverloaded. The nanofiltration membrane may further reduce organiccompounds dissolved in the fluid having a relative lower molecularweight than the organic compounds removed by such methods as organo-claysorbents. For some embodiments, removal of only relative highermolecular weight organic compounds without use of the nanofiltrationmembrane may be sufficient since the relative higher molecular weightorganic compounds result in more negative influence on the seleniumremoval unit 208 than the organic compounds with the relative lowermolecular weight.

Initial concentration of the thiosulfate dissolved in the fluid mayrange between 1 ppm and 1000 ppm, between 10 ppm and 100 ppm or about 40ppm. Reduction in the thiosulfate concentration within the fluid withthe thiosulfate removal unit 206 may result in the fluid having lessthan 1 ppm or less than 0.1 ppm of the thiosulfate. A reject streamseparated from the fluid by the thiosulfate removal unit 206 may mixwith effluent output from the selenium removal unit 208, be dischargedas waste, or undergo further wastewater treatment.

The selenium removal unit 208 at least reduces concentration of theselenium in the fluid. The sorbent material used in the selenium removalunit 208 for sorption of the selenium includes a substrate or supportthat may be a porous material and is associated with at least one oftellurium, selenium and sulfur. Examples of the support includeactivated or porous carbon alumina and/or silica. For example, a sulfurimpregnated activated carbon may define the sorbent material within theselenium removal unit 208. Percentage of sulfur loading of the supportranges in some embodiments from 1.0% to 40.0% by weight or from about10.0% to about 30.0% by weight. In some embodiments, methods of makingthe sorbent material may involve heating the support in presence of asulfur, selenium or tellurium compound, heating the support in presenceof elemental forms of sulfur, selenium or tellurium, wetting the supportwith a solution containing a sulfur, selenium or tellurium compound, orreacting a sulfur, selenium, or tellurium compound in presence of thesubstrate, such as a Claus condensation reaction. For some embodiments,solid catalyst or sorbent that is taken upon becoming spent from anotherprocess in which sulfur, selenium, or tellurium is involved provides thesorbent material still useful in the selenium removal unit 208 forsorption of the selenium.

The sorbent material displays affinity for selenium removal from thefluid. A flow path for the fluid contacts the sorbent material that ispacked within the selenium removal unit 208. The sorbent materialadsorbs the selenium within the fluid such that effluent from theselenium removal unit 208 provides the fluid treated to be suitable fordischarge into the environment or optional reuse.

In some embodiments, adjusting the pH of the fluid to between 1.0 and10.0, between 2.0 and 7.0 or between 2.5 and 5.0 facilitates adsorption.Adding to the fluid an acid selected from, for example, at least one ofsulfuric acid (H₂SO₄), tin(II) chloride (SnCl₂), iron(II) chloride(FeCl₂), aluminum chloride (AlCl₃), nitric acid (HNO₃) and hydrochloricacid (HCl) prior to contacting the fluid with the sorbent material canlower pH of the fluid that has an initial pH more basic than desired.Further, heating the fluid and/or the sorbent bed to between 1° C. and100° C., between 60° C. and 90° C. or about 75° C. may aid in theadsorption. Injection of steam into the fluid and/or heat exchange withthe fluid may raise a temperature of the fluid. Hydraulic flux of thefluid through the selenium removal unit 208 may vary from 0.1 to 20gallons per minute per foot squared (GPM/ft²), 1 to 6 GPM/ft² or from 2to 3 GPM/ft².

On an elemental selenium basis, the fluid may contain at least 0.01 ppmor at least 1.0 ppm of the selenium. For some embodiments, the fluidcontacts a sufficient quantity of the sorbent material such that thefluid contains at least 40% by weight less of the selenium content thanin the fluid entering the selenium removal unit 208. The fluid mayfurther contain an initial concentration of arsenic compounds, such asarsenite, arsenate, organo arsenic compounds and arsine, in a range of0.01 ppm to 10,000 ppm, 0.1 ppm to 4 ppm or 0.1 ppm to 1 ppm. Inaddition, the fluid may contain an initial concentration of mercurycompounds, such as elemental mercury, organo mercury compounds andoxidized mercury compounds (e.g., compounds containing dissolved mercurycations and/or mercuric chloride) in a range of 1 part per trillion(ppt) to 1 ppm or 10 ppt to 0.1 ppm. In some embodiments, the sorbentmaterial in the selenium removal unit 208 also sorbs such arsenic andmercury compounds from the fluid.

The polish treatment unit 210 at least reduces concentration of residualcomponents in the fluid. The fluid that exits the polish treatment unit210 or the selenium removal unit 208 may supply a process or applicationby, for example, being reused as oil refinery cooling tower water, waterinput into a boiler, desalter water or condensate water. The treatmentsenable removing the selenium without addition of chemicals such ascopper or iron that would present problems with reusing the fluid.Exemplary methods employed by the polish treatment unit 210 includephysical processes such as reverse osmosis, electrodialysis, and/orthermal methods to reject other constituents that may be undesired forfurther reuse.

FIG. 3 shows a system flow diagram of alternative functional fluidtreatments that may also form all or part of the selenium removalprocess. The treatments utilize a reducing agent injector 300 for addinga reducing agent to the fluid ahead of passing the fluid through aselenium precipitate remover 302. On an elemental selenium basis, thefluid may contain between 5 ppb and 1000 ppm or between 5 ppb and 2000ppb of selenite prior to treatment with the reducing agent injector 300and the selenium precipitate remover 302. In some embodiments, thereducing agent injector 300 and the selenium precipitate remover 302couple to receive the fluid from the selenium removal unit 208 shown inFIG. 2.

Examples of the reducing agent include compounds capable of generatingsulfite in solution. Such compounds include thiosulfate, bisulfite andsulfur dioxide. The reducing agent injector 300 adds enough sulfite tothe fluid to create a molar excess relative to selenite concentration inthe fluid. The molar excess may range from 1 to 100 or 10 to 20 timesthe selenite concentration. By way of example, the reducing agentinjector may introduce gaseous sulfur dioxide into the fluid or add asolution of sodium thiosulfate to the fluid. The reducing agent convertsselenite to insoluble elemental selenium forming precipitate in thefluid.

In some embodiments, adjusting the pH and/or temperature as discussedwith respect to the selenium removal unit 208 in FIG. 2 facilitatesselenium removal with the selenium precipitate remover 302. The seleniumprecipitate remover 302 may include sorbent material and operate and beanalogous to the selenium removal unit 208 described herein. Otheroptions for the selenium precipitate remover 302 include any filteringdevice, a settling tank and/or a bed filled with solid particles, suchas silica, alumina or activated carbon, without impregnation thereofwith sulfur, selenium, or tellurium.

FIG. 4 illustrates a block diagram for a first exemplary implementationof the selenium removal process. This implementation may be chosen insome embodiments for processing moderate levels of solids (about 500ppm) and relative low level of oils (>50 ppm) with treatment fordissolved organics (>400 ppm) and without treatment for thiosulfate. Afirst solids filter 400 and a second solids filter 402 sandwich a pH andtemperature adjuster 408. The filters 400, 402 exclude or preventpassage of particles above a certain threshold size. The pH adjustmentbetween the first solids filter 400 and the second solids filter 402enables removing precipitated materials, such as organic compounds thatare dissolved in the fluid until the pH of the fluid drops. After thesolids are removed, the fluid passes through a granular activated carbonbed 404 to reduce concentration of organic compounds that remaindissolved. The fluid then passes through two sulfur impregnatedactivated carbon beds 406 for removal of the selenium.

FIG. 5 shows a block diagram for a second exemplary implementation ofthe selenium removal process. Such implementation may be chosen forprocessing relative low levels of solids (<200 ppm) with treatment foroils (>100 ppm) and dissolved organics (>400 ppm) without treatment forthiosulfate. The implementation includes a filtration cartridge 500 toremove solid particulate from the fluid. Outflow from the filtrationcartridge 500 enters an oil sorbent bed 502 packed with organo-clay toremove the oil from the fluid. While the organo-clay in the oil sorbentbed 502 sorbs free and emulsified oil, the filtration cartridge 500 mayalso reduce oil content in the fluid by removal of oil coated solids,which are not as effectively removed by the oil sorbent bed 502. Oncethe solids and oils are removed from the fluid, the fluid passes to anactivated carbon bed 504 that sorbs the dissolved organics in the fluid.A pH and temperature adjuster 508 brings the fluid to a desiredtemperature and pH at any point ahead of the fluid being introduced intoa selenium sorbent bed 506 for removal of the selenium.

FIG. 6 illustrates a block diagram for a third exemplary implementationof the selenium removal process. Some embodiments with thisimplementation provide processing for relative high levels of solids(>500 ppm) with treatment for oils (>100 ppm), dissolved organics (>400ppm) and thiosulfate (>10 ppm). The implementation includes anelectro-coagulation unit 600 and ceramic membrane 602 used to removeoils and/or solid particulate from the fluid. Outflow from the ceramicmembrane 602 passes to an activated carbon bed 604 that sorbs thedissolved organics in the fluid. A nanofiltration membrane 606 disposedbetween the activated carbon bed 604 and a selenium sorbent bed 610removes thiosulfate from the fluid. A pH and temperature adjuster 608brings the fluid to a desired temperature and pH at any point ahead ofthe fluid being introduced into a selenium sorbent bed 610 for removalof the selenium.

FIG. 7 shows a block diagram for a fourth exemplary implementation ofthe selenium removal process. Embodiments with this implementation mayprovide processing for moderate levels of solids (about 500 ppm) andoils (50 ppm to 100 ppm) and for relative low levels of dissolvedorganics (<500 ppm) with no treatment for thiosulfate. A dissolved gasflotation unit 700 lifts particles and/or oil to a top of a tank forskimming off the particles and/or oil and may introduce an agent to helpfloat the oil such that the particles and/or oil are removed from thefluid. Outflow from the flotation unit 700 passes to an activated carbonbed 702 that sorbs the dissolved organics in the fluid prior to seleniumremoval with a selenium sorbent bed 704. A pH and temperature adjuster708 brings the fluid to a temperature and pH based on desired operatingparameters of the selenium sorbent bed 704. A reverse osmosis unit 706may remove remaining impurities in the fluid following use of theselenium sorbent bed. Such further treatment following the seleniumsorbent bed 704 while shown in FIG. 7 may be applied to any otherexemplary embodiments described herein.

FIG. 8 illustrates a plot of a first order rate constant for seleniumremoval versus water processed showing negative impact by oils onselenium sorption performance. A selenium sorbent bed was used to removeselenium from a petroleum refinery stripped sour water after only havinghad solid particulate matter removed from the water by using a 10 micronfilter before acidification of the water. The stripped sour waterexperienced upsets of elevated oil in water events and thus stillcontained oil upon being passed through the selenium sorbent bed. Theselenium sorbent bed fouled after treating only 1200 gallons of thewater.

FIG. 9 shows a plot of a first order rate constant for selenium removalversus water processed illustrating relative to FIG. 8 sorbent lifeextended by removal of soluble organics before contact of the water withsorbent for absorption of selenium even though the oils still influencethe selenium sorption performance. A selenium sorbent bed was used toremove selenium from a petroleum refinery stripped sour water afterhaving had both solid particulate matter and soluble organics removedfrom the water. The particulate matter was removed by using a 10 micronfilter before acidification of the water. The soluble organics wereremoved by passing the water through an activated carbon bed afteracidification of the water. The selenium sorbent bed fouled at least inpart due to oils remaining in the water after treating more than 11000gallons of the water.

FIG. 10 illustrates a bar graph of comparative undesirable increases inselenium sorbent bed lengths required to obtain a 95% selenium removalthreshold when the water contains various dissolved organic compounds.Given a relative bed length of 1 for treating the water without solubleorganics, concentrations of 6.3 millimolar acetate, propionate orbutyrate, 2.2 millimolar phenol or 2.2 millimolar cresol require therelative bed lengths greater than 1 to achieve the 95% selenium removalthreshold. For example, the relative bed length is greater than 4 if thewater contains the 2.2 millimolar cresol.

FIG. 11 shows a plot illustrating negative influence of thiosulfate onselenium removal due to additional mass transfer zone (MTZ) lengthneeded as thiosulfate concentration increases. The MTZ length increasesto achieve the selenium removal desired as the thiosulfate concentrationincreases from 0 to 70 ppm. For example, the MTZ length needed to removethe selenium from the water with 35 ppm thiosulfate doubles relative tothe MTZ length needed to remove the selenium from the water without anythiosulfate.

EXAMPLE 1

With reference to FIG. 4, a petroleum refinery stripped sour water wasfed to the first solids filter 400 and the second solids filter 402. Thefilters 401, 402 sandwiched the pH and temperature adjuster 408 operatedto adjust pH of the fluid to 2.75 from 6.0 and increase temperature ofthe fluid to 82° C. Solids entering the filters 400, 402 providing 20microns filtration were at a concentration of 500 ppm total suspendedsolids. Distribution of the particulates was 20 microns to 2000 microns.The pH adjustment between the first solids filter 400 and the secondsolids filter 402 enabled removing precipitated materials, such asorganic compounds that were dissolved above a pH of 2.75, resulting fromdropping of the pH. After the solids were removed, the fluid was passedthrough the granular activated carbon bed 404 to reduce concentration oforganic compounds that remained dissolved. The fluid was then passedthrough two beds of sulfur impregnated activated carbon 506 at a flux of2 GPM/ft². Percentage of the selenium removed was above 95% during timesof normal operation and for a period of more than 50 days without signsof selenium breakthrough.

EXAMPLE 2

To illustrate effectiveness of the reducing agent injector 300 and theselenium precipitate remover 302 shown in FIG. 3, water containingselenite at a concentration of 2 ppm was treated by adding thiosulfateas the sodium salt to the water so that a resulting solution was 100 ppmthio sulfate. The solution was adjusted to pH 2.5 using 0.1 molarsulfuric acid and heated to 68° C. The solution was then passed througha bed 100 centimeters long and 1 centimeter diameter of sulfurimpregnated activated carbon at a flow rate of 3 GPM/ft². The effluentselenium concentration from the sorbent beds was 6 ppb to 92 ppb andremained in this concentration range for the treatment of about 55liters of the solution. The steady state removal efficiency was greaterthan 95%. Under exact same conditions except with use of virginactivated carbon instead of sulfur impregnated activated carbon, theeffluent selenium concentration from the bed was about 15 ppb to 45 ppband remained in this concentration range for the treatment of about 44liters of the solution in order to provide a steady state removalefficiency greater than 97.5%. Changing thiosulfate addition to providethe solution with 10 ppm thiosulfate caused the effluent seleniumconcentration from the bed to be about 328 ppb to 972 ppb and to remainin this concentration range for the treatment of about 40 liters of thesolution in order to provide a steady state removal efficiency of about50%. Further, about 70% of the selenite fell out of a solution as aresult of settling prior to the solution even reaching the bed when thesolution contained 35 ppm thiosulfate and was otherwise at sameconditions. These results demonstrate that removal of the selenite byusing the reducing agent injector 300 may rely on simple solid fromliquid separation.

The preferred embodiment of the present invention has been disclosed andillustrated. However, the invention is intended to be as broad asdefined in the claims below. Those skilled in the art may be able tostudy the preferred embodiments and identify other ways to practice theinvention that are not exactly as described herein. It is the intent ofthe inventors that variations and equivalents of the invention arewithin the scope of the claims below and the description, abstract anddrawings are not to be used to limit the scope of the invention.

1. A selenium removal system for removing selenium from a fluid,comprising: a solids removal unit 200 for processing a fluid; an oilsremoval unit 202 coupled to receive the fluid from the solids removalunit; a soluble organics removal unit 204 coupled to receive the fluidfrom the oils removal unit 202; a thiosulfate removal unit 206 coupledto receive the fluid from the soluble organics removal unit 204; aselenium removal unit 208 coupled to receive the fluid from thethiosulfate removal unit 206; and a polish treatment unit 210 coupled toreceive the fluid from the selenium removal unit 208, the polishtreatment unit 210 to remove residual constituents that might otherwiseprevent recycling of the fluid.
 2. The selenium removal system accordingto claim 1, further comprising: a reducing agent injector 300 coupled toreceive the aqueous stream from the selenium removal unit
 208. 3. Theselenium removal system according to claim 1, further comprising: aselenium precipitate remover 302 coupled to receive the fluid from theselenium removal unit
 208. 4. The selenium removal system according toclaim 1, further comprising: a reducing agent injector 300 coupled toreceive the fluid from the selenium removal unit 208; and a seleniumprecipitate remover 302 coupled to receive the fluid from the reducingagent injector
 300. 5. The selenium removal system according to claim 1,further comprising: a sorbent tellurium material used in the seleniumremoval unit
 208. 6. The selenium removal system according to claim 1,further comprising: a sorbent selenium material used in the seleniumremoval unit
 208. 7. The selenium removal system according to claim 1,further comprising: a sorbent sulfur material used in the seleniumremoval unit
 208. 8. A selenium removal system for removing seleniumfrom a fluid, comprising: an electro-coagulation unit 600; a ceramicmembrane 602 positioned downstream of the electro-coagulation unit 600,wherein the electro-coagulation unit 600 and the ceramic membrane 602remove oils and solid particulates from the fluid; an activated carbonbed 604 located downstream of the ceramic membrane 602, wherein theactivated carbon bed 604 is a sorber of dissolved organics in the fluid;a nanofiltration membrane 606 located downstream of the activated carbonbed 604, wherein the nanofiltration membrane 606 removes thiosulfatefrom the stream; and a selenium sorbent bed 610 located downstream ofthe nanofiltration membrane
 606. 9. The selenium removal unit accordingto claim 8, further comprising: a pH and temperature adjuster 608located between the nanofiltration membrane 606 and the selenium sorbentbed 610, wherein the pH and temperature adjuster 608 brings the fluid toa desired temperature and pH before the fluid is introduced into theselenium sorbent bed
 610. 10. The selenium removal unit according toclaim 9, further comprising: a reverse osmosis unit 706 locateddownstream of the selenium sorbent bed 610, wherein the reverse osmosisunit 706 removes impurities in the fluid.
 11. A selenium removal systemfor removing selenium from a fluid, comprising: a dissolved gasflotation unit 700 that accepts the fluid and permits particles and oilto float on a top surface of the fluid; a skimmer for skimming off theparticles and oil from the top surface of the fluid; an activated carbonbed 702 located downstream of the dissolved gas flotation unit 700,wherein the activated carbon bed sorbs dissolved organics in the fluid;and a selenium sorbent bed 704 located downstream of the activatedcarbon bed
 702. 12. The selenium removal unit according to claim 11,further comprising: a reverse osmosis unit 706 located downstream of theselenium sorbent bed 610, the reverse osmosis unit 706 for furtherremoving impurities in the fluid.
 13. A selenium removal system forremoving selenium from an aqueous stream, comprising: a conditioningunit having at least one of an oil removal component, a soluble organicsremoval component, and a thiosulfate removal component coupled toreceive the aqueous stream for treatment thereof; and a selenium removalunit having a treated water output and an input coupled to receive anoutflow of the aqueous stream from the conditioning unit, wherein theoutflow is in fluid communication with a support impregnated with atleast one of sulfur, selenium and tellurium to absorb selenium in theoutflow within a flow path between the input and the treated wateroutput.
 14. The system according to claim 13, wherein the conditioningunit includes the oil removal component, the soluble organics removalcomponent, and the thiosulfate removal component that includes ananofiltration membrane disposed in a pathway of the aqueous streambetween the selenium removal unit and both the oil removal component andthe soluble organics removal component of the conditioning unit.
 15. Thesystem according to claim 13, wherein the conditioning unit includes theoil removal component that includes an organo-clay sorbent bed and thesoluble organics removal component that includes a guard bed with asorbent distinct from the support impregnated with at least one ofsulfur, selenium and tellurium.
 16. The system according to claim 13,wherein the conditioning unit as part of the thiosulfate removalcomponent includes a nanofiltration membrane that has a maximum poresize below 100 nanometers and is disposed between the selenium removalunit and a sorbent guard bed that forms at least one of the oil removalcomponent and the soluble organics removal component of the conditioningunit.